It is known to evaluate the so-called tinting strength of an individual pigment e.g. titanium dioxide or another white pigment. Such a method is disclosed e.g. in U.S. Pat. No. 2,253,551 (Booge) and involves providing standards by milling together a standard titanium dioxide pigment, refined linseed oil and varying amounts of ultramarine blue, the samples being graded by viewing the sample paste on a microscope slide. For a test pigment the colour strength is determined by the amount of the ultramarine blue to give a desired strength and such methods have become established in the art and survived for a long time, see e.g. U.S. Pat. No. 3,208,866 (Lewis) at column 6 lines 63-66.
It is known, however, that tinting strength of an individual pigment or of a group of pigments cannot be simply related to their performance in paint. This may be illustrated with reference to titanium dioxide which is the most important white pigment used in the coatings industry.
Titanium dioxide (TiO2) and other white pigments opacify paint films primarily by diffusely reflecting light. This reflection occurs because the white pigment scatters or bends light strongly. If there is enough white pigment in a paint film, almost all visible light striking it (except for a very small amount absorbed by vehicle or pigment) will be reflected, and the film will appear opaque, white, and bright. A change of refractive index promotes reflection and reflection of light will occur from the surface of TiO2 pigments with high refractive index (2.7) in contact with various coatings vehicles at low refractive index (e.g. about 1.5). Part of the light is refracted within the particles, and the higher the refractive index the shorter the path of the light within the film and the less depth of film needed to give a white rather than a grey colour when viewed over a dark background. Furthermore when the size of the TiO2 particles approaches half the wavelength of incident light, the particles can bend four to five times as much light as actually falls on them because a large amount of the light is diffracted when it passes close to the particles. In other words, the scattering cross section can be four to five times the geometric cross section of the particles. TiO2 is unique in that it combines high refractive index with a high degree of transparency in the visible region of the spectrum (although diffraction is also affected by volume concentration of the pigment and by “dry flat hiding” if air becomes incorporated into the film). One way to incorporate air into the paint in a stable manner is by adding porous materials which only reach their final optical properties in the dry state after the solvent has evaporated. This combination of properties affords the coatings formulator a route to highly opaque and bright whites or tints at minimum film thicknesses. For most efficient light scattering, the TiO2 pigment diameter should be slightly less than one-half the wavelength of light to be scattered. Since the human eye is most sensitive to yellow-green light (wavelength about 0.55 μm), the theoretical optimum particle size for TiO2 pigments for coatings is between 0.2 and 0.3 μm in diameter.
In addition to TiO2 and vehicle, many paints also contain extender pigments. These materials perform a variety of functions. White extender pigments are mineral compounds of relatively low refractive index and differ in composition, size and shape. They develop very little hiding in gloss and semi-gloss paints, but contribute dry-flat hiding (air-pigment interface) to paints at low cost and are used to control gloss, texture, suspension, and viscosity. The main types of extenders are carbonates, silicates, sulphates, and oxides. Their particle size ranges from 0.01 to 44 μm. High-gloss white paint usually contains only TiO2; a semi-gloss paint contains TiO2 and some extender pigments; a flat paint contains TiO2 but has a high extender content.
The particle size of TiO2 is small compared to the thickness of the film in which it is used. As discussed above. It has a theoretical optimum particle size between 0.2 and 0.3 μm, but as received by paint manufacturers is considerably larger because of the formation of agglomerates as a result of handling during the manufacturing process. The energy of simply stirring pigment into water or binder is not sufficient to overcome the particle attractive forces resisting the breakup of the agglomerates. By not minimizing the number of these agglomerates, the end-use properties will all be adversely affected. Although the pigment is designed to yield optimum hiding power, gloss and colour, these basic properties may not be realized if the initial dispersion of TiO2 is inadequate. The optimum dispersion for the pigment is defined where further grinding will not change its particle size distribution. A further problem is flocculation i.e. formation of loose clumps of TiO2 particles (i.e., flocculates) in a fluid system. Flocculation is often the result of an inadequate initial grind (dispersion), improper type or concentration of dispersant, pH mismatch, and temperature. Characteristically, these clumps are easily broken under moderate shear, but will quickly reform if the particles are free to move in the matrix. Flocculation can cause major problems, including loss of opacity and tinting strength.
Similar phenomena can occur with other pigments, whether white or coloured, and the colour strength and other properties of a pigment dispersion are not simply related to the nominal values of the pigment as supplied because it is essential to take into account agglomeration, flocculation and other phenomena to which the pigment may be subject.
US 2002/0174804 (Rodrigues) is concerned with making pigment dispersions that match a standard dispersion. It explains that it is important to carefully control these pigment dispersions with regard to tinting strength and colour through particle size adjustments as they are being made, so that when they are used in specified proportions to produce a desired paint, the load colour of the paint is easily shadeable/adjustable to an acceptable match to the standard colour for the paint. Acceptability of a grind was determined by traditional strength testing, which is a manual process that involves blending the dispersion with a standard white or black paint, spraying the blend onto panels, baking the panels and then comparing the panels to those of a standard batch of that dispersion blended with the same standard white or black using a spectrophotometer. Lightness differences between the dry sprayouts were then used as an indication of strength and acceptability of the grind. The improvement over traditional methods involved flowing the dispersion through a cell having a path length of e.g. 10-250 μm and viewing the dispersion in transmission at wavelengths from 400-700 nm. In embodiments the spectral transmission curve was measured and L*, a* and b* or other suitable colour values for the standard liquid dispersion which the dispersion being produced is to match were determined. Once the pigment concentration and relative strength had been determined, the process could also include analysing the spectral transmittance of the resulting dispersion to determine colour acceptability for use in finished paints where the dispersion is the prime dispersion or a significant component thereof. Even when strength was equal to that of the standard, the dispersion could be calorimetrically unacceptable owing to batch-to-batch pigment variability. In order to monitor these colour changes and indicate whether the colour is acceptable for use the spectral transmittance of the resulting dispersion was measured by the spectrophotometer and the L*, a* and b* colour values of the dispersion were then calculated from these measurements. A computer took these L*, a* and b* values and determined their differences from the L*, a* and b* values for the standard dispersion and from the magnitude of the numbers, determined the colour acceptability of the dispersion. However, the Rodrigues approach is only applicable to materials that can be viewed in transmission, and cannot be used for paint.
To the best of the applicants' knowledge and belief the Rodrigues technique has not found application in the paint industry. Currently the standard way of measuring colour strength or tinting strength of paint (as an example TiO2 based white paint or any other coloured paint) is as follows. A sample of such paint is mixed with a known, well characterized tinting agent of known colour in a defined ratio. The diluted sample is used to form a coating on a substrate which is then dried in an oven and measured with a lab-based colour measurement instrument. The tinting strength is then given by the amount of dilution necessary to generate a particular response from the colour measurement instrument (e.g. a particular set of L*, a*, b* values, describing the coordinates of the colour in a 3-dimensional colour space). The process is time consuming (a single test can take about two hours), lab based, relies on a specific, well characterized tinting agent and does not lend itself to automation.
White emulsion paints are commonly based on inorganic pigments such as titanium dioxide and extenders e.g. calcium carbonate, kaolin, talc, silica and mica. These ingredients are mixed e.g. batch-wise 5000 liters at a time with vinyl emulsion and other materials in large tanks. Checking the colour strength by traditional methods takes up to two hours for a single measurement and significantly slows the production process, since the information is needed to determine whether or not additional ingredients or other processing measures are needed to bring the batch within its intended specification. The same is true when tinting agents are added to the paint. A problem that is addressed by the invention is the provision of a method for checking the colour strength of paint or other generally non light-transmissive e.g. diffusely reflective materials which is simple to carry out and takes a shorter time than the traditional method. A further problem is to provide a test method which lends itself to automation.